Since my polymorph meme has only garnered three downvotes so far I thought I’d offer a bit more controversial take, and see if I can manage to stir the pot a bit with this one.
The touch of your shadow-wreathed hand can siphon life force from others to heal your wounds.
Doesn’t say “exclusively from others”. Without casting this spell you couldn’t do that normally.
It also says,
On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt.
No qualifiers about the target having to be a creature other than you. It just has to be a creature within your reach.
Also as regards:
As a DM there’s a few ways I’d consider to stop this being entirely game-breaking:
Do you also ban death ward and healing word? What about wizards who dip a level into virtually any other spellcaster or take a feat for a healing spell who can do this kind of thing without even having to make an attack roll or take necrotic damage?
The fact that it mentions others to begin with can’t be ignored though.
Essentially by including that gate in the spell we understand that Self is a valid target, but if target is self then heal does not occur.
There’s no need for the rest of the text to explain what has already been explained, it would be redundant.
I would rule of cool this to result in either instant death or not being downed, at least that way there’s high risk/reward.
There’s no need for that, wizards are strong enough as is. They can pick up cure wounds somehow and use that spell instead.
Doesn’t say “exclusively from others”. Without casting this spell you couldn’t do that normally.
It also doesn’t say that you cannot fly yet that doesn’t mean it gives you a flying speed. The spell only does what it says it does: It makes you able to siphon life force from others. In any situation not explicitly mentioned in the spell you are still bound by all of the other game rules and as far as I know there is no rule that’d say you can go siphoning life forces from anything without an effect explicitly stating that you can do so.
The D&D rules do not run on a principle of “you can do absolutely anything except for what is explicitly forbidden”.
No qualifiers about the target having to be a creature other than you. It just has to be a creature within your reach.
You do not get to cherrypick which sentences of the spell description you read and follow. It does quite explicitly state “can siphon life force from others to heal your wounds”. There is no concept of flavor text in 5e, every word of a spell description is rules.